This application claims the benefit, under 35 U.S.C. xc2xa7 365 of French Patent Application 02/06674, filed May 31, 2002.
The present invention relates to a planar antenna, more particularly to a multiband planar antenna of the slot type suitable for wireless networks, in particular for wireless networks operating in separate frequency bands.
In the scope of deploying mobile or domestic wireless networks, the design of the antennas is confronted with a particular problem which stems from the way in which the various frequencies are allocated to these networks. For instance, in the case of domestic wireless networks in the IEEE802.11a or Hiperlan2 standard, two separate frequency blocks operating in the 5 GHz band have been allocated to the various operators, as can be seen from the table below.
In order to cover both frequency bands, whether for a single standard or for both standards simultaneously, a variety of solutions have been proposed. The most obvious solution consists in using an antenna with a wide frequency band which covers both frequency bands at the same time. This type of wide-frequency-band antenna is generally complex in structure and high in cost. The use of a wide-band antenna also has other drawbacks, such as the degradation of the performance of the receiver due to the noise bandwidth and the jammer which can operate throughout the band covered by the antenna, this band also including the band unallocated to the specific applications which lie between 5.35 GHz and 5.47 GHz. Using a wide-frequency-band antenna involves more stringent filtering constraints for the transmitter, in order to comply with the out-of-band transmission power masks or profiles, namely the maximum powers which are allowed to be transmitted inside the allocated band, but also outside this band. This leads to additional losses and extra cost for the equipment.
In wireless networks, at a given instant, the antenna furthermore covers a channel having a width of about 20 MHz, lying in one or the other of the two bands. One solution making it possible to avoid the drawbacks associated with wide-frequency-band antennas might be to use an antenna whose frequency band can be tuned electronically.
Planar antennas which, as represented in FIG. 1, consist of an annular slot 1 operating at a given frequency f are also known, the slot being fed by a feed line 2. More precisely, on a substrate consisting of a usual printed circuit metallized on both of its faces, the annular slot 1 which may be circular in shape, but which may also have any other closed shape, is produced conventionally by etching on the side intended to constitute the earth plane of the antenna. The feed line 2 is intended to feed the slot 1 with energy by electromagnetic coupling. For example, it consists of a line produced in microstrip technology, which is positioned on the other side of the substrate from the slot 1 and is oriented radially with respect to the circle which forms this slot, in the embodiment which is represented.
In this embodiment, the microstrip line-annular slot transition of the antenna is produced in a known fashion so that the slot 1 lies in a line short-circuit plane, that is to say in a region where the currents are strongest. Hence, lm=kxcexm/4, where xcexm is the wavelength being guided in the line and k is an odd integer. The length lxe2x80x2m is chosen in order to achieve 50 xcexa9 matching of the line 2. In this case, the perimeter p of the slot 1 is chosen to be equal to a multiple m of the wavelength being guided in the slot, m being a positive whole number. Hence, P=2xcfx80R=mxcex, where xcex is the wave length being guided in the slot. In this case, the resonant frequencies of the various modes are in practice multiples of the frequency f, these modes corresponding to the fundamental mode, the higher mode etc.
An antenna of this type can hence be modelled around its resonant frequency f by a parallel RLC circuit, such as represented in FIG. 2. The relationship LCxcfx892=1 is therefore obtained at the resonant frequency, with xcfx89=2xcfx80f, f being equal to the resonant frequency.
The antenna described above offers the particular advantage of having a compact structure and of being easy to produce. It is furthermore known to the person skilled in the art that the equivalent circuit of a diode, in particular a PIN diode, is a capacitive circuit when the diode is in the OFF state or an inductive circuit when the diode is in the ON state.
The present invention therefore relates to an improvement to planar antennas of the annular slot type, which makes it possible to provide coverage of a plurality of frequency bands while avoiding the drawbacks and difficulties associated with wide-frequency-band antennas.
The present invention hence relates to a planar antenna carried by a substrate including a slot consisting of a closed curve dimensioned in order to operate at a given frequency and fed by a feed line positioned so that the slot lies in a short-circuit plane of the feed line, characterized in that it includes, in parallel on the slot, a plurality of switching means capable of assuming a closed state or an open state so as to modify the central frequency and the width of the operating frequency band of the planar antenna.
The switching means preferably consist of a diode or a varactor allowing continuous adjustment of the frequency. According to an alternative embodiment, a diode is at least put in parallel with a varactor. Furthermore, the switching means is or are fitted in parallel, as a function of the resonant frequency desired for the antenna, between the electrical short-circuit plane for the slot, giving a minimum value, and the electrical open-circuit plane for the slot, giving a maximum value.